Cholinergic neurons are the first neurons to die during Alzheimer's disease. As a consequence therapeutic strategies to treat the early cognitive symptoms of this disease have included cholinesterase inhibitors; in fact a positive allosteric modulator of the a7 nicotinic acetylcholine receptor (a7n-AChR) has recently entered phase III clinical trials. How these cognitive enhancing therapies achieve therapeutic benefit is unclear; however, data suggest that there is an important downstream modulation of NMDA receptor (NMDAR) function. Our preliminary data lead us to postulate that cholinergic activity, acting through the astrocytic a7n-AChR, modulates the release of astrocyte-derived D-serine, which in turn acts on the co-agonist site of the NR1 subunit of the NMDAR to augment NMDAR function. According to this hypothesis the well-known night/day time alterations in cholinergic activity drive oscillations in D-serine accumulation and thus NMDAR function. We have four specific aims to test our hypothesis. Aim I: Is the amount of D-serine available to synaptic NMDARs regulated by a sleep-homeostasis mechanism? Aim II: What are the physiological and behavioral consequences of daily fluctuations of D-serine availability? Aim III: Does an astrocytic source of D-serine contribute to wakefulness dependent regulation of D-serine? Aim IV: Does astrocytic a7n-AChRs drive wakefulness dependent modulation of D-serine? To achieve these aims we will use brain slice electrophysiology and amperometric biosensors in situ as well as in vivo recordings of D-serine. In addition to performing studies in wild type mice we will use molecular genetics to make cell type specific manipulations of gliotransmission and of the a7n-AChR and will use astrocyte specific optogenetics to selectively stimulate the release of D-serine from this glial cell sub-type. Defining the role of nicotinic receptor modulation of astrocyte-derived D-serine, and the consequent effects on NMDARs, synaptic plasticity and behavior, will provide new insights into the mechanisms of action of cholinergic therapeutics and will provide a new opportunity to understand the regulation of D-serine, a critical co-agonist of the NMDAR.